Prevention of hard scale formation in oil wells



Patented Aug. 23, 1932 TUNITED STATES PATENT OFFICE JOHN'C. WALKER, F ELDORADO, KANSAS, ASSIGNOR TO EMPIRE OIL AND @EFINING COMPANY, OF DOVER, DELAWARE, A CORPORATION OF DELAWARE PREVENTION OF HARD SCALE FORMATION m OIL WELLS No Drawing.

The present invention relates to the prevention of hard scale deposits in oil wells and more particularly to a method .of preventing corrosion of and the formation of scale deposits on metal parts in oil wells.-

Many pil wells experience serious and expensive losses of material and time because of the formation of deposits of hard scale on the metal tubing and casing in the ,well

and also because of corrosion of the, metal parts by the liquids present in the well. An investigation of the Waters associated with oils in the oil producing localities has dis- .closedethat the compounds which are chiefly' responsible for the corrosion of the metal parts installed in oil wells are hydrogen sulfide, organic sulfide and/or sulfur; or sulfuric acid, carbonic acid or other mineral, acids associated with oxygen. All

of these compounds are active in destroying iron and steel parts. It has been found that when these underground waters contain hydrogen sulfide a reaction takes place in the well resulting in the formation of iron sul- 2 fide. Ferrous sulfide thus formed tends to cling to the metal parts especially in spots where there is an impurity in the iron or steel. The iron sulfide thus present in contact with the metal parts of the well sets up an electric couple when immersed in the salt water or electrolyte which is present in the well. The current set up by this couple passes from the iron sulfide to the iron at the point of contact, thence from the iron to the electrolyte, (salt water and hydrogen sulfide) carrying iron into solution, and thence back to the iron sulfide, completing the circuit. Corrosion has been found by experiment to be many times more rapid in the presence of ironsulfide in contact with iron due to this electrolytic action.

Many oil wells in addition to experiencing corrosion of the tubing and deposition of the iron sulfide or soft scale are also'"- troubled withthe formation simultaneously of a hard scale which forms on the tubing as a very tough and very hard deposit greatly resembling boiler scale. Hard scale deposits sometimes aforr'n coatings of nearly, an inch in thickness in a comparatively short time,

Application filed October 30, 1926. Serial No. 145,880.

and in wells in which such deposits are encountered the pulling and replacing of the tubing becomes avery expensive job. The extra weight of the tightly adherent scale adds materially to the strain on the joints of the metal tubing, which are also weakened by corrosion. This added strain due to the weight of the scale coupled by weakened joints causes frequent 'and very expensive parting jobs. Thus the deposition of a oneeighth inch thickness of scale will increase the weight of a string of tubing approximately 30% and the upper jointsof the string must thus sustain this additional weight (about one and one-half tons in the case of a twenty-five hundred foot string). These deposits of hard scale materially shorten the lifeof the tubing because the scale deposited on the upper. part of the working barrel ofiers strong opposition to the removal of the travelling valve and cups from the barrel and necessitates the use of extra pull, which in turn also causes frequent parting jobs on the strings. Moreover these hard scale deposits greatly interfere'with the proper operation of the pumpin apparatus and in many cases where the tu ing has been removed for cleaning it is unfit torerun for the reason that the scale isso hard and clings so tenaciously to the tubing that it is almost impossible to'dislodge it by ordinary means (as boiler flue cleaners, hammers and the like).

One object of the invention is toprovide a methodfor preventing the losses in oil well operation caused by the corrosive action of the liquids in the well on the metal of the easing, tubing and pumps installed in the well. To accomplish this object one feature of the present invention contemplates the prevention of corrosion of the metal parts in oil wells by lowering the hydrogen ion concentration of the brines or waters associated with the oil in the well to a point necessary to afford protection of the metal. To accomplish this, compounds which have an alkaline reaction in water are preferably introduced into thewell in a way to bring them in contac with .the brines or water in the well.

Another important object of the present infine 7 vention is to provide a methodfor preventing the formation of scale deposits on the metal parts of oil wells,thereby avoiding the losses mentioned above.

It has been found that wells which give the greatest trouble due to the deposition of hard scale are generall those which have casing leaks. An analysis of the hard scale deposits formed in such wells has shown that in most cases the deposits consist chiefly ofstrontium, calcium or barium sulfates. Apparently these deposits of insoluble alkaline earth sulfates are formedin the following manner The top waters encountered in the wells, that is, the waters in sands above the oil producing sands and which are presumably shut off from the producing sand by the casing, may for example carry varying amounts of stron tium chlorid (which is a soluble salt) in solution. When a leak developsin the casing due to corrosion or some other cause these top waters find their way into the oil producing sand. The waters in the producing sand may carry soluble sulfates which are absent in the top waters. These soluble sulfates react with the soluble strontium chlorid carried in the top waters to form insoluble strontium sulfate. ,Ordinarily the precipitation of strontium sulfate on the metal parts in the well would be expected to take place rather rapidly with the formation of a granular or pulverulent precipitate.- However, due to the high concentration of chlorid ions both in the top waters and in the waters associated with oil inthe producing sands, and also due to the comparatively small excess of sulfate ions, the strontium sulfate which is formed crystallizes out slowly and deposits in the form of a very hard and compact crystalline scale. 4 a.

While the hard scale de osits encountered in the wells of certain oil elds are made up largelv of strontium sulfate, barium or calcium sulfates and likewise the insoluble carbonates of these alkaline earth metals are also found to cause serious trouble due to halfd scale deposition in wells in other localities. Likewise it has been found that in some 10- calities the soluble alkali sulfates maybe present in the top waters and the soluble chlo-. rides in the bottom waters of the well- Since in general the hard scale deposits appear to occur inv oil wells only when there is mixing of water produced with the oil or from below the oil horizon with water from the upper sands which are presumably shut off by the casing (water string), the most obvious method of preventing the formation of this scale would appear to be to completely exclude the top water or the bottom water from the well. However, this obvious method of treatment is not always possiblev -or practicable. It is only in exceptional cases that it is possible to completely shut off water produced from below the oil sand as as hemlock, oak or the like.

by plugging. It is never possible to completely shut off by plugging waterproduced with the oil in the oil sand. It is frequently possible'to obtain a complete shut off of top water when the casing is not seriously corroded and when there 1s a good casing seat.

'' and it is therefore extremely expensive and.

hazardous to attempt to shut off .water 'by either of the methods outlined above, artic ularly as the results are'not sure and t e well ma be lost. 1

other object of the present invention is to provide a comparatively cheap and economical method of preventing scale formation and of allowing economical production in oil wells when a water shut off such as that above referred to is not practicable.

To. accomplish this object another feature of the invention contemplates treating the well liquids to avoid precipitation of insoluble alkaline earth salts whenever possible, and in cases where such treatment is not possible effecting the rapid precipitation of the insoluble salts and further treatin the precipitates formed to prevent the bui ding up of scale on the metal, parts of oil wells and to allow for removal of the precipitated matter from the well by suspension in the water and oil pumpedfrom the well.

With these and other objects andfeatures in view the invention consists in the method of preventing corrosion and hard scale formation in oil wells hereinafter described and particularly defined in the claims.

Essentially the method for treating oil wells'to prevent formation of scale deposits and corrosion constituting the present invention in its preferred form comprises adding to the liquids in the well a rea ent comprising a mixtureof a neutralizlng agent for lowering hydrogen ion concentration and a material capable of reacting with the soluble alkaline earth salts resent in the liquids of the well to form a llght, flocculent and non-adhering or non-scaling precipitate. The preferred reagent. consists of a mixture of caustic soda and a tan bark extract such Other reagents suitable for use in accordance with the meth- 0d of thepresent invention are the .alkaline solutions of tannic acid and similar organic acids or compounds suchas tan bark extracts of chestnut, oak-and similar materials and the alkaline reacting solutions of .sodium aluminate, sodium silicate and similar inorganic compounds capable of reactin -with magnesium, calcium, strontium and arium salts to produce light, fiocculent and even gelatinous preci itates.

The amount 0 reagent which it is necessary to employ and also the composition of the reagent and the proportions of alkaline compound and of flocculating compound employed in making up the reagent are determined by the volume and analysis of the water associated with the oil in the particular well to be treated, by the volume production of the vwell, and by the character and amount of corrosive and scale-forming compounds respectively present in the we] Under certain conditions it has been found a that the formation of the precipitates of insoluble alkaline earth salts tending to form hard scale deposits in oil wells can be prevented by a modification in the method outline above. Thus if the bottom or top waters of the well under theatment contain a low sulfate and a comparatively low chlorid concentration, for instance and if the top (or bottom) waters contain strontium chlorid and sodium chlorid, for instance, in relatively large proportions, the mixture of the two waters may just barely result in the slow precipitation of insoluble strontium sulfate. Under such conditions it has been found that the addition of a concentrated brine (such as sodium chlorid) in relatively small proportions will serve by building up the chlorin ion concentration to force back the dissociation of the strontium chlorid to the point where the comparatively low concentration of SO ions is insufiicient to cause principitation ofundissociated strontium sulfate. Thus this modified treatment results practically in increasing the solubility of strontium sulfate by the addition of brine to the mixture of top and bot-- the precipitate material under treatment were calcium sulfate and would be less applic'able, though still possible under special conditions in the case of barium sulfate, because of the relative solubilities of the three salts.

In a well where barium) and sulfate ions are present in their respective waters in relatively large amounts, but at the same time in quantities nearly proportional to their reactin weights and in the presence of relatively igh chlorine ion concentration, the same slow crystallization- ,of the alkaline earth sulfates will take place It is believed that the strontium (calcium,

when the waters are mixed, and this'slow crystallization will result in cementing and hard scale formation. To meet this situation another modified method oftreatment has been devised which comprises adding sodium sulfate to the well water mixture. The addition of sodium sulfate increases the concentration of S0 ions and results in driving back the dissociation of the insoluble strontium or other alkaline earth sulfate,- with the resultant decrease in the solubility of this compound and its rapid precipitation. The tendency under these conditions will be toward'the formation of a finely divided pulverulent precipitate. of strontium sulfate which will not have as great tendency to grow on the tubing or on other crystals to form a scale. This lessened tendency to form scale is further aided by addition of the preferred caustic soda-tannin (sodium tannate) reagent. The two reagents may be applied in any suitable way, as by mixing the sodium tannate or similar reagent with the sodium sulfate in a solution of suitable concentration for treatment.

In'treating one well it has been found that a rea 'ent comprising thirteen parts by weight of sodium hydroxid to one part of tannic acid, when addedto the well in proportions of one hundredth of one per cent by weight of the daily water production of the Well, is entirely satisfactory both in inhibiting the corrosive action of the well waters and in preventing the deposition of calcium and strontium carbonates and sulfates on the metal parts. Ex-

tan bark extract by itself is not effective in preventing the deposition of strontium sulfate, but' that when the tan bark extract is admixed with caustic soda in the proportions of '10 lbs. of the powdered extract (60% or better soluble tannins) to 2 lbs. of thesodium. hydroxid, and this mixture made up to a volume of about twenty barrels by solution in fresh or salt water, the treatment of the well over a. twenty-four hour period with that amount of reagent prevents completely the deposition of hard strontium sulfate scale and is also effective in preventing extensive "corrosion in the well.

At another well a reagent comprising two partsby. weight of caustic soda and one part by weight of tannin extract (60% soluble tannins) dissolved in water to 25'35 B. has given satisfactorv results. At this well the treatment consisted in adding four to five gallons ofthe above reagent per day continuously to the wellwaters by means of a specially designed treating device stationed at the casing-head. The strong solution is dripped continuously into the casinghead and-washed down by a fine stream of salt water from the well v(about'three barrels per day). Another reagent successfully used under'conditions of'greater corrosive action and a somewhat less marked scaling tendency consisted in ten parts by weight of caustic soda and three parts by weight of tannin extract.

The questionas to whether the reagent shall 7 the treating agent necessarily results in exrather than the continuous method of carrytensive periods in which there is a low percentage neutralization of the corrosive comounds ofthewell liquids, in general it has een found-;that in the long run the periodic I ing out the treatment is most economical. In

general it has been found-that it is not neces saryto maintain complete neutralization of the acidic compounds of the well liquids for the full twenty-four hours of the day, particularly in wells which are only pumped'durhours. It has been found that the scale cleansing efl'ect occurring during the short periods of high neutralization immediately after the periodic" addition of concentrated reagent to the well liquids leadsto'the formation of rotective :Eerrous hydroxide films which a ord. protection, with the result that the ill effects of low" neutralization for the hours elapsing between additions oi treating agent. are minimized. i .The caustic alkalis-in general, and particu-' larly caustic. soda, are very strong neutralizing agents'for the acidic compounds present in oil well brines. The caustic alkali component bf thepreferred typesof reagent used in accordancewith the present invention is therefore efiective in preventing corrosion.-

The tannic acid and tan bark extracts are not;ii 1 general effective by themsleves ascorrosion preventing agents, in fact under certaiij' conditionsthey themselves cause corrosion', but w en, they are neutralizedwith caustic soda in excess the resulting mixture hi, -been found to be very effective not only ,iiifpreventing corrosion but also inpreventc'ium, strontium or barium sulfates and carbonateson the metal'parts in the well.- Morejoverthere'agents used in this process are in general very eflz'ective inpremoving soft ironfsulfi'd and also non-sulfid deposits from the iron tubing and dispersing-this material in the form of a suspension in the fluid in the well, so. that the iron sulfide or non-sulfide material is thereafter removed with the fluids pumped from the well. Likewise the present reagents are very effective in causing the formation of light, flocculant non-adherent eas es" fluid. The non-scaling precipitates formed from the alkaline earth salts of the brine by reaction with the present reagents also have a tendency to envelo or be absorbed on any alkaline earth sul ate or carbonate crystals or on iron sulfide particles which may iron sulfide scale is treated by the present process for the first time, the water pumped from the well after treatment is usual y black with suspended iron sulfide which is so finely divided as to be practically in colloidal suspension. This pumping ofblack water from the well may proceed for several days, but ing the normal working day of eight or ten if the treatment is continued it will be 0bserved that the.water gradually loses color a as' the scale of iron sulfid is removed.

When treating the liquids in the well according to the present invention it has been found that it is necessary for best results that 'the treating agent difl'use rapidly through the liquids in the well. This result is accomplished according to the present in vention-by'adding the treating agent to a brine or salt water which has a speclfic gravity substantially the same as the specific gravityof the water in the'well to be treated. In making up a reagent the desired s ecific gravity for thepurposes herein descri ed is obtained either by adding a salt such as sodium chlorid, which is stable under the con ditions in the well, or by using water from the well to be treated. In the latter case a calculated excess of caustic soda is added to compensate for the calcium salts precipitated by the caustic or other neutralizing compound used.

The treating agent may be addedto the well'in various ways. The determination of I jthe manner of adding the treating agent de ingthefleposition of the hard scales of calpends largely on the number of hours that the well is pumped each day. Thus the procedure generally adopted in treating wells which are pumped from two to twelve hours per day consists in adding the calculated amount of treating agent as a strong solution to the well just after the shut down of the pump, and washing down the chemical with about one barrel of either fresh or salt water.. Wells which are operated twentyfour hours a-day are provided with a special form of treating device consisting essentially in a chain bucket pump driven from the crank shaft or other moving part of the well and designed to feed strong chemical in regularly distributed amounts throughout the pumping period. The chemical is washed to ing the the bottom of the well with a fine stream of" salt water from the lead line. For treating wells having a pumping period of ten to twelve hours a day'a combination method of treatment is generally used, comprising the continuous addition of a regulated small amount of chemical during the course of the pumping period followed upon shut down by an additional. charge which is washed down with salt or fresh water in amount sufficient to provide treatment for the incoming water at thebottom of the well durperiod in which the pump is idle.

It has been found that cups, whether of leather or composition, have a much longer life in wells treated by the method of the present invention, since only soft fine grained precipitates are formed which are pumped out of the well readily without injuring or scoring the cups or the barrel. The cups in wells which are not treated often last but a short time, due to abrasive action of the gritty and granular products of corrosion and the hard scale flakes or chips formed on the metal parts adjacent the cups and jarred loose during the operation of the pump. The

barrel of the pump and the cups are not lubricated by the present chemical treatment but the cups and working barrel are subjected to service under conditions substantially normal to a well not troubled with corrosion orhard scale formation. The life of the cups, balls, valve seats and barrels in wells treated in accordance with any of the methods outlined above is noticeably increased thereby and the savings due to such increased life are large.

It is often the case that the oil wells which give the greatest trouble due to corrosion and to the formation of hard scale deposits on the tubing and casings, are also the source of much trouble and expense due to the formation of stable emulsions of the oil-inwater type, commonly known as BS emulsions. Accordingly one feature of the present invention which is sometimes employed in the treatment of wells subject not only to corrosion and to the formation of hard scale deposits but also to the formation of BS emulsions in pumping, consists in admixing with the treating agent hereinbefore described a small amount of cresylic acid or similar compound having a lowering effect on the surface tension of the water forming the dispersed phase of such BS emulsions and having consequently a breaking action on the emulsion. The proportions of neutralizing agent or alkali, tan bark extract and cresylic acid or other emulsion breaking agents which are used in preparing the reagent for'treating a given well are determined in each case by analysis of the well liquids and by tests carried out to determine the nature of the emulsions formed on pumpin In some of the claims t e expression chemical hard scale inhibitor has been used as a generic term to define chemical reagents of the type above described (such as a caustic solution of tannin or certain alkali metal salts) which when "added to oil well liquids are capable of reacting with or changing the character of insoluble alkaline earth salts or other potential hard scale-forming components of the said liquids, as above described, so as to produce light and flocculent nonscale-forming precipitates, or so as to inhibit the formation of hard scale-forming precipitates.

The invention having been thus described, what is claimed as new is:

1. The method of treating oil well liquids to render them non-corrosive and non-scaleforming which comprises adding to the well liquids a reagent comprising a mixture of an alkali metal salt and a caustic alkali solution of tannin in amounts suflicient to cause precipitation of the insoluble alkaline earth salts from the well liquids in a pulverulent and non-scale-building form.

2. A method of treating oil well liquids to render them non-scale forming consisting of adding to the liquids a reagent comprising a salt water solution of an alkali and a tan bark extract in quantities and proportions sufiicient to effect 80% to complete neutralization of the acidity of the oil well liquids and to prevent hard scale deposition of calcium, magnesium, strontium and barium sulfates and carbonates.

3. A method of treating oil well liquids to render them non-scale-forming comprising mixing with the well liquids an alkaline brine solution of a tan bark extract in which the brine has substantially the same specific gravity as the well water.

4. A method of treating oil well liquids to prevent the formation of hard scale deposits on metal parts in contact with the liquids which comprises mixing with said liquids an alkaline solution of a tan bark extract.

5. The method of treating oil well liquids to render themnon-scale-forming comprising adding a strong caustic alkali solution of tan bark extract to the well and washing the agent down the well'with water.

In testimony whereof I afiix my signature.

' JOHN C. WALKER. 

